In typical wavelength division multiplexing (WDM) communications, multi-channel optical signals are simultaneously transmitted through a single optical line. When the optical signals are transmitted over several hundred kilometers, loss is generated every several tens of kilometers. Thus, optical amplifiers should be installed at intermediary positions on the line to compensate for the loss.
To measure gains of multitude channels of a WDM erbium doped fiber amplifier (EDFA) which is used as optical fiber for WDM, source channels are necessary to input optical signals of each channel. For example, to measure gain of an EDFA with respect to an 8-channel optical signal, eight units of signal sources and a measurement system capable of detecting the optical signal are required. It is difficult, however, to provide all the necessary sources and it becomes expensive to do so.